A. Field of the Invention
The present invention relates to containers for storing and transporting twist drill bits. More particularly, the invention relates to a drill bit case that is re-configurable to store and protect drill bits of various types and sizes.
B. Description of Background Art
The manufacture of most Printed Wiring Boards (PWB's) of the type used to hold and electrically interconnect electronic circuit components requires the drilling of many small holes through the board. The holes are required for receiving component leads, or to constitute tunnels, the walls of which are subsequently electroplated to form an electrically conducting passageway or vias between various layers of the PWB. Most PWB's are fabricated as laminated stacks of conductive copper foil sheets adhered to alternating insulating sheets made of a material such as fiberglass, the latter containing glass fibers solidified with a resin such as epoxy. Conductive surface pathways on the PWB are formed by etching away portions of the copper foil layers. The glass fibers in the insulating layers of the PWB are highly abrasive, and can quickly dull drill bits used to drill holes in the PWB. Thus, most drill bits used in the manufacture of PWB's are tipped with carbide or a similar refractory material, which is very hard, but also brittle.
A typical PWB has a thickness of about 0.062 inch, and has hundreds of holes drilled through it. Frequently, holes are conveniently drilled simultaneously through a stack of vertically aligned PWB's, the stack consisting of two to five PWB's. Each contact of a drill bit with the upper PWB in a stack is referred to as a hit. Because the abrasive material of PWB board materials dulls typical drill bits after about 3,000-5,000 holes are drilled, the drill bit must be removed from service and re-sharpened after about 1,500-2,000 hits, i.e., after only as few as a dozen or so PWB's have been drilled. Thus, it can be appreciated that the manufacture of even modest quantities of PWB's requires frequent removal of worn or dull drill bits from a drilling machine at a drilling station where the PWB's are drilled, transport of the drill bits to a re-sharpening location, and transport of new or re-sharpened drill bits to the drilling machine station. PWB drill bits are typically small, having a standard shank diameter of about 3.175 mm, and a standard length of about 38.1 mm. Moreover, the diameter of the fluted, front cutting portion of a PWB drill bit, equal to the size of the diameter of the hole to be drilled, is usually quite small, in the approximate range of 0.6 mm down to 0.3 mm or less. Thus, typical PWB drills bits are fragile, being subject to breakage or chipping if either the fluted cutting lips or point of the bit impact another bit or another hard object. For this reason, PWB drill bits are generally stored and transported in some sort of protective container, which may, for example, utilize Styrofoam or other such cushioning material to protect the sharpened lips and points of the drill bits from being broken, cracked, chipped or dulled. Such containers desirably accommodate a plurality of drill bit stands, each holding a plurality of vertically oriented, longitudinally spaced apart drill bits. The drill bit stands are employed to facilitate automatic loading and unloading of drill bits from automatic drilling machines. Some PWB drill bits have annularly-shaped, depth control rings attached to the shank of the bit. The front annular surface of the ring is located at a pre-determined distance rearward from the point of the drill bit, and during automatic drilling operations, abuts a fixed structural element, thus limiting the insertion depth of the drill bit to a pre-determined value. Whether or not fitted with an insertion depth limiting ring, the length of the fluted front cutting portion of PWB drill bits varies to suit a particular application. Thus, because of the variety of sizes and configurations of PWB drill bits, presently available drill bit cases are generally not capable of suitably containing and protecting all of the various drill bits commonly used in PWB manufacturing.
Although it is stated in patent report No. 978216 that the invention disclosed therein can store both ringless drill bits and drill bits with rings, the case disclosed requires use of different drill stands, and requires a change of some parts of the case to accommodate drill bits of various types. The present invention was conceived of to provide a drill bit case which may store various types and sizes of PWB drill bits, without requiring exchange of parts of the case.